Field of the Invention
The invention concerns a method for determining absolute three-dimensional reception sensitivity maps for reception coils in a magnetic resonance apparatus, in particular in a magnetic resonance apparatus having a basic magnetic field strength of at least 3 T, in the presence of a subject under examination that affects the reception sensitivity. The invention also relates to a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
Inhomogeneities in the magnetic resonance fields used to acquire raw data in a magnetic resonance scanner are a common cause of image artifacts in the magnetic resonance image. The field inhomogeneities, which result in image inhomogeneities, cause a particular problem when the basic magnetic field (B0 field) has a field strength of at least 3 T. If a subject under examination, in particular a patient, is brought into the magnetic resonance scanner and hence into the basic magnetic field, electrical effects arise that may influence the transmission and reception properties of the RF coils used, and are visible as shading in the reconstructed magnetic resonance datasets. This shading can conceal the underlying anatomy or make it harder to discern, and is hence detrimental to a reliable diagnostic assessment.
The reason why there is a sharp increase in the image artifacts resulting from inhomogeneities in the RF field (B1 field), produced by the RF coils, at higher basic magnetic field strengths, in particular field strengths of at least 3 T, is that in such strong fields, the wavelengths at the Larmor frequency become comparable to the dimensions of the subject under investigation, in particular a patient, and the electromagnetic properties of human tissue affect the ultimate distribution of the RF fields inside the body, which results in the inhomogeneity side-effects in magnetic resonance imaging.
The shading problem can be thought of as having two separate components: one component that arises in the radiation of the RF pulses via transmit coils (TX), and another component arising from the influence of dielectric effects on the reception behavior, i.e. the reception sensitivity, of the reception coils. In the transmit case, the dielectric effects interact at high field strengths to produce an inhomogeneous distribution of the emitted RF power inside the body, which results in erroneous flip angles in the imaging volume and hence in unwanted modulation of the original magnetic resonance signals in the tissue by the sine of the flip angle. As noted above, these effects are far weaker at lower field strengths, for example at 1.5 T, and can be ignored. In order to combat these transmit (TX) effects for higher field strengths, for example field strengths of at least 3 T, the prior art proposes parallel transmission (pTX) techniques for the emission of RF pulses for the purpose of pre-calibrating the transmit profile of a number of transmit antennas, i.e. to measure the emitted B1 fields (which are then often referred to as B1+ maps). Such data are then used to pre-distort the emitted RF fields so that then a homogeneous excitation, i.e. a homogeneous distribution of the flip angles, occurs in the target area from which raw data are to be acquired.
As also noted, however, the same dielectric effects also affect the reception component of the magnetic resonance examination because the reception sensitivity of the reception coils (often referred to as B1− maps) depends on the originally unknown anatomy that is meant to be imaged. The effects are primarily determined by the distribution of the electrical conductivity σ and the permittivity (dielectric constant) ε in the tissue.
The latest magnetic resonance imaging techniques are essentially based on having coil sensitivity information available that is as accurate as possible in order to be able to correct RF-related image homogeneities. It is precisely in the receive case (RX), however, that often only relative reception sensitivity maps are available for reception coils. If the relaxation-free signal intensity at a specific position is considered for a single transmit/reception coil (TX/RX coil), it normally depends not only on the tissue contrast and the flip angle, but also on the positive (for TX) and negative (for RX) circularly polarized components of the transverse RF magnetic field. The main obstacle to determining absolute transmit/reception sensitivity maps lies in the interdependence of the transmission sensitivity (described by the B1+ map), the reception sensitivity (described by the B1− map) and the image contrast, as given by the equations known in the prior art. It should be noted here that in addition, the flip angle of course depends on the magnitude of the RF transmission field (and hence on the transmit sensitivity). Known techniques measure only the magnitude of B1+ but not the phase of B1+.
It would be possible to solve this problem using an enhanced technique in which the entire arrangement of coils and subject under examination is rotated with respect to the direction of the basic magnetic field (B0 field). The z-component of the RF field for the transmit and receive sensitivities could then be introduced into the equations. For such a case, there are mathematical analyses that show that six rotations are needed to determine the entire RF vector field and hence to determine the absolute transmission and reception sensitivity information. Such an approach, however, is obviously unsuitable for examining patients in the patient receiving area of a magnetic resonance scanner of a magnetic resonance apparatus.
The use of reception sensitivity maps for reception coils becomes particularly important in acceleration techniques used in parallel imaging, known as accelerated PAT (parallel acquisition technique). Such acceleration techniques are based on using multiple reception coils in parallel (pRX) that have very different reception sensitivities or, in the ideal case, receive sensitivities that do not overlap spatially. This immediately results in the problem that all PAT image reconstruction techniques require prior knowledge about the actual receive sensitivities of the reception coils. As was explained, however, these reception sensitivity maps also depend on the anatomy to be imaged, in particular the electromagnetic properties thereof It is assumed in the prior art that ultimately it would be impossible to encode the absolute receive sensitivities of the reception coils and the magnetic resonance signals produced by the originally unknown anatomy in such a way that it would be possible to determine absolute reception sensitivity maps.
This is why known PAT reconstruction techniques such as SENSE or GRAPPA use an automatic estimate of relative receive sensitivities of the reception coils, i.e. the ratio of the individual-coil reception sensitivity and a common reference denominator, for example the square root of the sum-of-squares (ROOT-SOS) of all the individual coil sensitivities. In the SENSE technique, the relative reception sensitivity maps are determined in the image domain, whereas the GRAPPA technique uses an implicit and relative relationship between k-space coil sensitivities.
Whereas the approach that uses relative reception sensitivity maps for the reception coils may be adequate for suppressing PAT aliasing artifacts caused by undersampling, the disadvantage with this approach is that the reconstructed magnetic resonance image dataset is still modulated by the reception sensitivity of the (possibly virtual) reference coil, which itself suffers from inhomogeneities. This results in an uneven presentation of the imaged anatomy, i.e.
an inhomogeneous brightness, so that some regions are incorrectly shown as emitting weaker magnetic resonance signals. Such RX shading artifacts even arise when almost-perfect, homogeneous flip-angle distributions are achieved by parallel transmission (pTX).
Two approaches for reducing these artifacts are known in the prior art. One approach proposes providing a post-processing step for the reconstructed magnetic resonance image dataset, which is known as “prescan normalization”. In this approach, two additional low-resolution magnetic resonance images are acquired, one using the local reception coils, the other using a whole-body coil operated in reception mode. The main assumption in this correction approach is that the reception sensitivity of the whole-body coil is practically homogeneous over the entire imaging volume. The dielectric effects in magnetic resonance scanners having basic magnetic field strengths of at least 3 T mean, however, that this assumption is erroneous because even for the whole-body coil, reception sensitivity profiles differ widely between different patients and between different areas of examination.
In a second approach, which is known only for acquisitions in the head region, it is assumed that the absolute reception sensitivity maps (B1− maps) can be determined by mirroring the B1+ maps with respect to the central sagittal plane. Hence the underlying assumption is that the head is right-left symmetrical. This is not generally true, however, which means that errors occur not only when the head is not centrally positioned but also when abnormalities are present such as tumors, stenoses or hemorrhages. In addition, this approach is also subject to further known limitations in the estimate of B1+ maps, namely the need for a number of acquisition processes (scans), which are susceptible to movement, low signal-to-noise ratios and low resolutions. Moreover, the mapping of the transmission sensitivities of the transmit coils supplies only the strength of the B1+ field, while the phase is not generally known and is affected by the phase of the reception chain, eddy-current errors and B0 inhomogeneity.
Hence particularly in the field of PAT image reconstruction techniques such as SENSE or GRAPPA, there is the desire to have available, instead of the conventional relative reception sensitivity maps of the reception coils, with which shading artifacts still occur, absolute reception sensitivity maps, which would make it possible to obtain magnetic resonance image datasets that are free of RF shading artifacts. There are also other applications, however, in which complete knowledge about the RF field or the underlying properties could be useful, for example for SAR quantification.